Certainly don't rely on school - DCs need plenty of support from home too.
I would start by contacting the school - if they are good then you can get their advice on where the weaknesses are and work with them.
As far as home based support goes then study guides are really helpful but start by making sure DCs notes from school are all up to date so you are revising rather than covering new material.
For 'factual' subjects then making and constant revision of 'flash' cards is a great idea.
For subjects that require more application of understanding eg. maths, then constant practice of material (eg questions in revision guides) is also helpful.
When do they have the mock exams?
Its a good idea to try to revise as much as possible for these as they will give a better idea of where any problems lie and what is going well.
It is certainly a good idea to start now - particularly with making sure material is all in place and everything has been understood.
Christmas holidays are a good opportunity to make headway with revision and after that if the school do not set practice papers then make a start with some each week at home.
By the Easter holidays the goal should be to be doing past papers and going over revision cards rather than starting a panic programme of learning (this is the ideal but rest assured there will be many 16 year olds for whom Easter marks the start of the revision process)
Try to take it calmly and avoid too much stress - slow and steady wins the race